PROJECT SUMMARY The proposed studies in lampreys build on the findings that both jawless (agnathans) and jawed (gnathostomes) vertebrates have prototypic T-like and B-like lymphocyte lineages, although the jawless vertebrates (lampreys and hagfish) generate variable lymphocyte receptors (VLR) for antigen recognition by recombinatorial conversion of incomplete germline VLR genes (VLRA, VLRB and VLRC) into fully assembled VLR genes using diverse leucine-rich-repeat (LRR) donor sequences. Analysis of the development, distribution and function of the lamprey VLRA and VLRC lymphocyte lineages suggest that the VLRA and VLRC lineages represent the agnathan T cell analogues of gnathostome ?/? and ?/? T cells with allorecognition responsibility. The serial multistep assembly of VLRC and VLRA genes coincides with expression of cytidine deaminase1 (CDA1) a thymus-equivalent gill region termed the thymoid, whereas VLRB and CDA2 expression occurs primarily in hematopoietic tissues. Surprisingly, three additional CDA1-like genes, the products of which have deaminase and mutagenic activity, have now been identified. The first specific aim is to define the multigene CDA1 family members in lampreys, determine their cellular expression patterns and elucidate the structure and functions of their protein products. Circumstantial evidence implies that the VLRA and VLRC receptors rely on associated transmembrane molecules for their cell surface expression and signaling competence. The second specific aim is therefore to determine the composite receptor structure and signaling competence for VLRA and VLRC. Pilot studies indicate that the VLRA+ and VLRC+ lymphocytes preferentially respond to allogeneic white blood cells. The third specific aim is to identify the antigens recognized by VLRA and VLRC receptors with the related goal of identifying lamprey histocompatibility antigens. Candidate histocompatibility genes will be examined for genetic polymorphism, expression patterns and immunostimulatory potential. A long term goal is to test the hypothesis that VLRA and VLRC recognize histocompatibility-associated antigens to achieve self versus non-self discrimination.